Asbestos has been widely used hitherto in such application fields where heat resistance, anti-flammability and alkali resistance are required as protective tools such as fire fighting clothes, working clothes before furnaces and sheets protecting welding sparks, sealing materials such as gaskets and ground packings, heat-shielding materials, filtering materials represented by bag filters, abrasive materials such as brakes and clutches, electric insulating materials and cement products cured in an autoclave.
However, it has been clarified that asbestos remarkably injures human health, for example, it causes cancer of the lung and the use of asbestos has been therefore becoming to be legally restricted or prohibited in many countries.
A variety of fibers such as carbon fiber, steel fiber, glass fiber, aramide fiber, phenolic fiber and flame resistant fiber (oxidized fiber) have been therefore proposed as the materials substituting for asbestos.
Among these substituting fibers, an acrylic oxidized fiber prepared by heating and oxidizing an acrylic fiber in air as being disclosed in the specifications of U.S. Pat. Nos. 3,508,874 and 3,961,888 most attracts notice because it has low density and excellent anti-flammability and it is flexible and relatively cheap. However, the acrylic oxidized fiber above described has an unhomogenuous structure where the degree of oxidation of the fiber skin layer is much larger than that of the core and therefore has low tensile strength and poor toughness. Spinning, weaving and knitting of the fiber is difficult and even if they can be done, abrasion resistance of the product thus obtained is poor. Heat resistance of the product is not good either and, for example, there was a problem that the strength decreases and the practical performance disappears when it is used at a higher temperature than 150.degree. C. for a long time.
Therefore, sufficient performance could not be always obtained for such applications substituting for asbestos as heat resistant/flame resistant protective tools, packings, filters and electric insulating materials where heat resistance, flame resistance and abrasion resistance are required. A brake lining based on the above described acrylic oxidized fiber is proposed in the specification of U.S. Pat No. 4,259,397, but its abrasion resistance and heat resistance at higher temperature are poor and there are such problems in its practical performance that cracks occurs in the abrasive material during the usage and the durability is not always enough.
Moreover, high strength acrylic fibers and polyvinyl alcohol fibers are tried to use as reinforcing fibers for hydraulic materials such as cement, which is one of the main application fields of asbestos, but they do not endure a strong alkali at higher temperature in such a field that curing in an autoclave is carried out in steam at 180.degree. C. and therefore completely lose their reinforcing effects. Acrylic oxidized fibers prepared by heating and oxidizing the above described acrylic fibers in air do not show improved alkali resistance at higher temperature either and they can not be therefore used for this application. Meanwhile, an acrylic sulfide fiber obtained by heating an acrylic fiber in sulfur dioxide is described in Japanese Patent Publication No. 36461/1972 (corresponding to BP 1,282,500) as an example of the methods for preparing special carbon fibers and it is described there that this sulfide fiber has a tensile strength higher than the acrylic fiber before sulfuration.
However, according to the results obtained by the present inventors when an acrylic fiber prepared of an acrylonitrile polymer having an ordinary degree of polymerization and containing a copolymerizing component used for manufacturing a conventional acrylic fiber such as, for example, acrylic acid, methacrylic acid, itaconic acid, methyl methacrylate and acrylamide, prepared by the same procedure as the example 1 in the above described publication, is heated and sulfurized in sulfur dioxide, the denier of the sulfurized fiber obtained increases due to the introduction of sulfur bonds. As the result, increase in the tensile strength can not be obtained and the tensile elongation remarkably decreases. The improvement of the toughness can not be expected either. It is therefore clarified that the problems related with spinning, weaving and knitting and abrasion resistance can not be solved by this method in a similar manner as the oxidized fiber obtained by heating and oxidizing the above described acrylic fiber in air. Therefore, the sulfide fiber described in this known example is only useful as an intermediate substance in manufacturing carbon fibers and the fiber satisfying the characteristics substituting for asbestos can not be obtained.
The present inventors have been intensively studying to improve these problems and thereby reached this invention.